Ruby Team

Hi I've just started playing pokemon ruby and this is my current team:Marshtomp-Tackle,Bide,Water Gun,Mud Shot;Swellow:Double Team,Steel Wing,Wing Attack,Quick Attack;Vigoroth: Scratch,Yawn,Fury Swipes,Slack Off;Nincada-For Cut and Flash.
I am actually at the 3 gym at Wattson.What should I add to my team,a fighting type,etc...?

A fighting type would work well against Wattson since he has one Magnemite and a Magneton. However, you already have your Marshtomp, which is part Ground-type, this gym should not be a problem for you. :)
But since you are close to the Fiery path after this gym, I would get a Fire-type for long-term. I recommend Numel. It is a part Ground-type so you can counter the gym leader after Wattson, Flannery.
You could also get a Magnemite or Voltorb later on when New Mauvile opens up, which is after you beat Norman. Either one will be a good addition to the team, espcially against Winona and Wallace later on.
I hope this helps! ^^

Looking ahead to end-game, here is my humble suggestion for a team that will cover all of your bases for the E4, working in who you already have on your team :)


Swampert:
-Surf
-Earthquake
-Really, anything you want for the other two slots


Slaking:
-Hyper Beam (you're going to be resting every other team, anyways, right? : P)
-Shadow Ball (Ghost is physical in this gen, and Ghost-type mons won't be able to hit back :P)
-Yawn
-Again, anything that you want


Swellow
-Aerial Ace
-Return/Secret Power
-Double Team
-Steel Wing/Fly (if you don't want to carry around an HM slave for it)

For fighting types I'll give you some options: Medicham or Hariyama. Meditites are only obtainable about 3/4 of the way through the game, so you might not want to wait that long. Hariyama's a great early alternative.


Medicham:
-Hi Jump Kick
-Psychic
-Calm Mind
-Fire/Ice/Thunder Punch (from Move Relearner)

or


Hariyama:
-Brick Break
-Seismic Toss
-Fake Out
-Bulk Up

Rest of the team:


Magneton (New Mauville):
-Thunderbolt
-Tri-Attack
-Thunder Wave
-Anything


Walrein (Spheal found in Shoal Cave):
-Ice Beam
-Surf
-Body Slam
-Encore


Also, in my opinion, it might be best to just drop Vigoroth/Slaking, as it is IMHO just excess weight. If you decide to do that, I would go with Manectric over Magneton, as it has access to Bite to take care of Pheobe. My Manectric set:


Manectric:
-Thunderbolt
-Thunder Wave
-Bite
-Anything